Deck Joist Span Calculator
Pick your joist size and spacing, enter the span you need, and see whether the joist works — or what to switch to. Values follow IRC 2021 Table R507.5 for Southern Pine #2.
Result
- Max span for 2x8 at 16" o.c.13'7"
- Your required span12 ft
- VerdictOK — joist size works
- Remaining margin1.58 ft
How it works
IRC 2021 Table R507.5 (Southern Pine #2, 40 psf live load)
- 2x8 at 16" on center spans at most 13'7" per IRC 2021 Table R507.5
- Your span = 12 ft
- 12 ft ≤ 13'7" → OK
How the deck joist span calculator works
Span tables come from the residential code. They account for the wood species and grade, the spacing, and the expected loads: 40 pounds per square foot of live load (people, furniture) plus 10 psf of dead load (the deck's own weight).
Taller joists span further — depth resists bending far better than thickness. Going from 2×8 to 2×10 adds roughly 3 feet of allowable span at 16" spacing.
Closer spacing also increases allowable span: a 2×8 reaches 15'0" at 12" on center but only 11'1" at 24".
Formula
IRC 2021 Table R507.5 (Southern Pine #2, 40 psf live load)
Worked example: can a 2×8 span 14 ft?
- 2×8 at 16" on center spans at most 13'7" per IRC 2021
- Required span = 14 ft = 14'0"
- 14'0" > 13'7" → too long by about 5 inches
- Options: move to 2×10 (max 16'4"), drop to 12" spacing (max 15'0"), or add a mid-span beam
Max span — Southern Pine #2, 40 psf live / 10 psf dead
| Joist | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|
| 2×6 | 11'7" | 10'6" | 8'7" |
| 2×8 | 15'0" | 13'7" | 11'1" |
| 2×10 | 18'0" | 16'4" | 13'4" |
| 2×12 | 21'0" | 19'0" | 15'7" |
IRC 2021 Table R507.5. Other species and grades differ — verify with your local building department.
Frequently asked questions
How far can a 2×8 deck joist span?
Up to 13'7" at 16" on center, 15'0" at 12" and 11'1" at 24", for Southern Pine #2 with standard 40 psf live load per IRC 2021. Other species and grades differ.
How far can a 2×6 deck joist span?
Up to 10'6" at 16" on center. A 2×6 works for small decks or walkways with a mid-span beam, but most 12 ft+ spans need a 2×8 or larger.
Does joist spacing affect span?
Yes, significantly. Narrower spacing means each joist carries less deck area, so it can span further. The same 2×8 spans 15'0" at 12" spacing but only 11'1" at 24".
Can I trust these numbers for my permit?
They mirror IRC 2021 tables for one common case: Southern Pine #2, 40/10 psf. Your jurisdiction may use a different code edition, and your lumber grade may differ. Submit the tables approved by your local building department.